This invention relates generally to methods and apparatus for well logging, and more specifically relates to methods and apparatus for determining volumetric fractions and flow rates of individual phases in multi-phase flow regimes.
In producing wells it is no uncommon to find the well fluid flow regime consisting of multiple phases, such as oil and water, oil and gas, or oil, water and gas. Often, one or more of these phases is an undesired element in the well production flow. For example, in the case of a well fluid flow regime consisting of oil and water, the oil is typically the fluid phase desired to be produced and the water is typically an undesired phase in the production flow. When the degree of water present in the well production flow becomes excessive, logging surveys are run at a plurality of depth locations within the well to facilitate the determining of the flow rates of the individual phases at each of the locations. From these flow rate determinations, which will yield information regarding the depth locations and rates of water entry, remedial actions to control such water entry may be chosen.
Measurement of the flow rates of the individual fluid phases is complicated by the fact that not only do the individual phases of the flow regime flow at different velocities, referred to as phase slippage, but also the nature of the flow pattern of the phases is not uniform throughout cross-sections of the pipe. This non-uniformity of the flow pattern is caused by one or more of a multiplicity of phenomena which are known in the art, such as, for example, stagnation, heavy-phase fall-back, and circulation, and is accentuated by such factors as large pipe, low flow rates, and/or deviated boreholes. Although the volumetric fractions of the individual fluid phases as determined across cross-sections of the well, also known as the phase holdups, are not uniform, they do bear a functional relationship to the flow rates of the individual phases across such cross-sections of the well, the exact nature of such functional relationship being dependent upon the conditions under which the fluid phase volumetric fractions were determined. Therefore, logging surveys to determine individual phase flow rates typically include measurements of the volumetric fractions represented by the individual phases.
So as to achieve a maximum of reliability of the volumetric fraction and flow rate determinations, it is desirable to determine the well flow characteristics while the well is actually producing. This is because an interruption of the well production may cause alterations in the flow characteristics of the well, including water entry, for which it is difficult or impossible to anticipate and/or compensate.
The oil and gas industry has attempted to determine the volumetric fractions of the individual fluid phases within these producing wells by conducting logging operations to determine either the density or the dielectric response of the well fluid. One means by which these determinations have been attempted has been by intersecting the fluid flow regime with the appropriate logging instrument while allowing the fluid flow to continue around the instrument. It can be appreciated, however, that this type of logging operation only determines the density or dielectric response of such portion of the fluid flow regime as actually engages the measuring system of the logging instrument. Therefore, fluid phases which do not intersect the instrument, or non-uniformities in the flow pattern caused by effects such as those described previously herein, which occur in the flow regime may cause the readings from the logging instrument to yield less than optimal data as to the nature of the fluid flow regime. Additionally, the accuracy of this type of surveying may be further complicated by the unknown effects upon the multi-phase flow regime when a logging tool is introduced into the producing well.
Further difficulties arise in determining the correct volumetric fractions of the fluid phases once the density or dielectric response measurement is obtained. Because the average density of the well fluid is generally the volumetrically proportional average of the densities of the individual phase components of the fluid flow regime, the density of the well fluid varies in a generally linear functional relation to changes in the volumetric fractions of the individual phases in the fluid flow regime. The fluid density measurement, however, typically offers a less than optimal degree of resolution of the individual phase volumetric fractions when the well fluid is composed of certain fluids, for example, water and oil, partially because of the relatively similar densities of the fluids, approximately 1 for water and 0.8 for oil, at surface conditions. In contrast to this, a fluid capacitance instrument which measures the dielectric response of the fluid, such dielectric response being directly related to the dielectric constant of the fluid, offers a measurement of a relatively high degree of resolution of the phase volumetric fractions present in the measured fluid due to the relatively disparate dielectric constants of water, approximately 78 at surface conditions, and of oil and gas, approximately 3 and 1, respectively, at surface conditions. This simple dielectric response measurement is difficult, however, to correlate to accurate phase volumetric fractions because the conductivity and dielectric properties of some fluids, including oil and water, are known to vary substantially with temperature. Further, the presence of other fluids or dissolved solids within the well fluid may alter the dielectric response of the well fluid. Therefore, calibrations of the fluid capacitance instrument dependent solely upon characteristics observed under surface conditions may lack validity when related to measurements taken within the borehole environment. Additionally, the dielectric properties of a mixture of oil and water or gas and water have been determined to be not always a linear reflection of the volumetrically proportional average of the relative dielectric response characteristics of the two fluids.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a method and apparatus by which a fluid dielectric response measurement may be interpreted in view of survey conditions, thereby facilitating a functional determination of the volumetric fractions and flow rates of individual fluid phase components within a multi-phase flow regime.